The term Volatile Organic Compounds (VOCs) refers to a range of carbon-based chemicals. They can be found in vapor or liquid form.
There are various sources of VOC pollution in indoor air: the "occupants" through breathing and body surface, cosmetic products or deodorants, heating devices, cleaning materials and various products (e.g. glues, adhesives, solvents, paints), clothes recently treated in laundries, cigarette smoke and work tools, such as printers and photocopiers.
Other important sources of pollution are building materials and furnishings (e.g., furniture, carpeting, upholstery). High concentrations of VOCs are found in the periods immediately following the installation of the various materials or the installation of furniture.
Emissions of VOCs are highest at the beginning of the product's life and tend to decrease considerably over fairly short periods of time (from one week for paints and adhesives, to six months for other chemical compounds). The exception is formaldehyde, which tends to show relatively constant releases over many years.
Finally, the incorrect positioning of air intakes in proximity to highly polluted areas (e.g. high traffic routes, underground parking lots, garages) can result in significant penetration of VOCs from outside.